As the capacity and functionality of integrated circuit microelectronic devices increase, the integration of microelectronic elements is also increased. In other words, a number of microelectronic elements fabricated per unit area of a microelectronic substrate is increased. Accordingly, manufacturing techniques must be able to provide high resolution patterns. In particular, dry etching techniques such as plasma fusion etching have been used to achieve high resolution patterns.
Plasma etching processes, however, can be difficult to control. When etching a layer of a microelectronic structure, important characteristics of the etching process include the etch profile, the etch selectivity with respect to a lower layer of the microelectronic structure, the etch rate, and the etch uniformity. These etch characteristics depend primarily on the characteristics of the etching apparatus, and the process gases. In particular, the etch uniformity is primarily dependent on the characteristics of the etching apparatus, while the etch profile, etch selectivity, and etch rate are primarily dependent on the process gases used.
One technique for improving the etching profile is to add a gas which forms a polymer to the gas mixture. The added gas allows the plasma etching process to achieve high resolution patterns. In particular, this technique has been described in "VLSI Technology" by S. M. Sze (2nd Edition, McGraw Hill Press, 1988, pp. 200-204), and U.S. Pat. No. 4,490,209 entitled "Plasma Etching Using Hydrogen Bromide Addition" to Hartman.
When a layer of a material including silicon is etched according to conventional plasma etching techniques, a halogen compound containing fluorine (F) or chlorine (Cl) is provided as a main etching gas. The choice of the compound used for the main etching gas is dependent on the properties of the layer to be etched, the desired etching profile, etching selectivity with respect to another layer of the microelectronic structure, and compatibility with other process gases. Additional process gases can be mixed with the main etching gas. These additional gases have predetermined functions. In particular, inert gases such as helium (He) and argon (Ar) with relatively heavy masses, can act as a carrier for the main etching gas. These inert gases may also aid in etching by physical sputtering. Oxygen (O.sub.2) and nitrogen (N.sub.2) can be provided and converted to a radical state (O and N) or converted to an ionic state (O.sub.2.sup.+ and N.sub.2.sup.+) by the plasma discharge. The radical and ionic states of oxygen and nitrogen can be used to control the etching profile by increasing or decreasing the polymers which are generated during the etching process.
In addition, hydrogen bromide (HBr) can be provided and dissociated in the plasma discharge. Bromine can thus be adsorbed on sidewalls of the etched portions of the layer thus forming a Si--Br polymer line. This Si--Br polymer line can act as a passivation layer for the sidewalls adjacent the etched surface. FIGS. 1A and 1B are photographs illustrating a polysilicon layer which has been etched by a conventional plasma etching technique using a mixture of gases including chlorine, hydrogen bromide, oxygen, and argon. As shown in FIGS. 1A and 1B the profile of the etched polysilicon layer is approximately a reverse trapezoid, and the etched perpendicular plane is tilted.
Conventional plasma etching methods may thus be unable to provide sufficiently perpendicular etching profiles. Accordingly, there continues to exist a need in the art for improved plasma etching techniques.